## Xto 4th power -3X to 3rd power - 40x squared = 0

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### Xto 4th power -3X to 3rd power - 40x squared = 0

Xto 4th power -3X to 3rd power - 40x squared = 0 How do I set this up into a quadratic equation? There are no examples close to this in my book. Do I sub. U forX to 3rd? If can have the set up or a formula for working this I can get it.
buddy

Posts: 132
Joined: Sun Feb 22, 2009 10:05 pm

### Re: Xto 4th power -3X to 3rd power - 40x squared = 0

buddy wrote:Xto 4th power -3X to 3rd power - 40x squared = 0 How do I set this up into a quadratic equation? There are no examples close to this in my book. Do I sub. U forX to 3rd? If can have the set up or a formula for working this I can get it.

I take it that the equation given is $x^4-3x^3-40x^2=0?$ The first thing you should do is see if you can find any common factors in the terms on the left-hand side. What can you factor out of $x^4-3x^3-40x^2?$

Once you have the left side factored completely, set each factor equal to zero (separately) and solve for $x.$ Since this is a fourth-degree polynomial, you will have at most four solutions.

Good luck!
Reckoner

Posts: 1
Joined: Sat Mar 21, 2009 11:56 pm

### Re: Xto 4th power -3X to 3rd power - 40x squared = 0

do i do x2 out f everything? then wat?
buddy

Posts: 132
Joined: Sun Feb 22, 2009 10:05 pm

Yes, factor the x2 out.

. . . . .$x^2\left(x^2\, -\, 3x\, -\,40)$

Hint: What factors of 40 are three units apart?

stapel_eliz

Posts: 1717
Joined: Mon Dec 08, 2008 4:22 pm

### Re: Xto 4th power -3X to 3rd power - 40x squared = 0

got it thnx anyway
buddy

Posts: 132
Joined: Sun Feb 22, 2009 10:05 pm